A common problem in audio processing is that an information-bearing signal is disturbed by one or more sinusoidal signals. A conventional method for suppressing interfering signals is to use fixed notch filters tuned to the frequency of the sinusoidal interference, as described in “Halbleiter-Schaltungstechnik” by Ulrich Tietze and Christoph Schenk, Springer, 12th edition, 2002, which is incorporated by reference herein in its entirety.
For notch filtering, in order to cause only a slight degradation in the signal of interest, the filter's notch is required to be very sharp, and for a good suppression the frequency of the interference needs to be known precisely. If this is not the case, the usual method of notch filtering is no longer effective and an adaptive approach has to be used, as proposed in “Adaptive IIR Filtering in Signal Processing and Control” by Philip A. Regalia, Marcel Dekker, 1994, which is incorporated by reference herein in its entirety. In this approach, the filter synchronizes with the main sinusoidal interference that contains the most power and suppresses it completely. The filter is also able to track minor time-dependent changes of the interference frequency. However, the approach has a major drawback in that it does not preserve the spectral content of the information-bearing signal at the notch frequency. A clean separation of two sinusoids, one representing noise and the other representing useful information, is thus not possible.